1. Field of the Invention
The present invention relates to a spot welding system and, in particular, to a spot welding system including a spot welding gun that welds workpiece to be welded under pressure between a movable electrode tip driven by a servomotor and a stationary electrode tip facing the movable electrode tip.
2. Description of the Related Art
In a spot welding system, an operation of calibrating the relationship between a torque command provided to a servomotor which drives a spot welding gun and welding pressure generated by the torque command at the distal ends of electrode tips is performed. The operation is performed before starting welding. Thereafter, the spot welding gun repeats the operation of welding workpieces to be welded under pressure according to an operation program.
The spot welding gun includes a movable electrode tip and a stationary electrode tip. The movable electrode tip depresses the stationary electrode tip to elastically deform a metal arm to which the stationary electrode tip is attached, thereby generating required welding pressure between the electrode tips. However, repeatedly applying high welding pressure for a long period of time can cause local plastic deformation of the arm or local cracks in the arm because of metal fatigue.
When local plastic deformation or local cracks occurs in the arm of the spot welding gun, the stiffness of the entire arm of the spot welding gun decreases. Accordingly, elastic displacement quantity of the arm of the spot welding gun increases. For this reason, workpiece to be welded can be depressed to degrade the weld quality even when welding pressure is not changed. Furthermore, continuing to use an arm having cracks can cause the arm to further develop cracks and eventually break. Therefore, an operator needs to visually inspect the conditions of the spot welding gun at regular intervals. However, it takes time for the operator to visually check spot welding guns individually and, in addition, it is difficult for an inexperienced operator to detect minute plastic deformation or minute cracks.
Moreover, even when the stiffness of the entire arm of the spot welding gun has not decreased, a mechanical unit of the spot welding gun, for example, ball screws, bearings or the like can become worn or lubricant in a mechanical unit can deteriorate. If this is the case, the frictional resistance of the mechanical unit will change.
For example, as the frictional resistance of a mechanical unit increases, the transmission efficiency of the mechanical unit decreases. Accordingly, even when a torque command that has been calibrated in the past so that desired welding pressure is generated is used, there can be a deviation between actual welding pressure and desired welding pressure.
If that is the case, actual welding pressure generated in accordance with the torque command provided to the servomotor needs to be detected with a pressure sensor and the relationship between the actual welding pressure and the torque command needs to be recalibrated. However, when welding pressure is measured with the pressure sensor, the spot welding system needs to be shut down. Therefore, the calibration operation may not be performed while welding workpieces.
For this reason, actual welding pressure measurement with the pressure sensor after the spot welding system being shut down is likely to be performed at prolonged intervals. Consequently, the spot welding system may be kept in operation without the operator being aware of a deviation in welding pressure, resulting in weld quality degradation. Moreover, since the pressure sensor is relatively expensive, it may be difficult to provide a large number of pressure sensors.
Various methods for solving these problems have been proposed for spot welding guns equipped with a servomotor.
For example, Japanese Laid-open Patent Publication No. 2007-29994 discloses that a regression formula is obtained from the relationship between pressure application time and the position of the distal end of an electrode tip when a number of pressure application operations have been performed beforehand and, when there is a significant departure from the regression formula, the spot welding gun is determined as deteriorated.
In Patent Publication No. 3503359, an elastic displacement quantity of an electrode tip measured by an encoder of a servomotor is multiplied by a predetermined electrode tip support stiffness to obtain the actual welding pressure. Then, a welding pressure coefficient is corrected so that set welding pressure becomes equal to the actual welding pressure.
In Japanese Laid-open Patent Publications No. 2008-296226 and No. 2010-000528, a change in frictional resistance in a mechanical unit of a spot welding gun is detected based on a current flowing through a servomotor in a steady operation state and the change is corrected to correct welding pressure.
However, when a crack occurs in the arm of the spot welding gun in Japanese Laid-open Patent Publication No. 2007-29994, the position of the distal end of electrode tip will change. Accordingly, when determining an abnormality in the spot welding gun based on the relationship between the position of the distal end of the electrode tip and pressure application time, it can be faultily determined that the spot welding gun has not deteriorated even if the gun has actually deteriorated. There is another problem that when a deviation in the position of the distal end of the electrode tip occurs due to wear or deformation of the electrode tip, the accuracy of the diagnosis decreases.
In Patent Publication No. 3503359, the position of the distal end of electrode tip when the electrode tip is abutted is determined by taking advantage of the fact that motor current increases when the electrode tip abuts on another electrode tip. However, to accurately determine the position of the distal end of the electrode tip, the electrode tip needs to be caused to abut on the other electrode tip at low speed. Moreover, welding pressure generated when the electrode tips are brought into contact at low speed differs from the actual welding pressure of the spot welding gun at the time of manufacture. This decreases the accuracy of a detected welding pressure deviation.
Furthermore, the position of the distal end of an electrode tip changes due to wear and deformation of the electro tip during welding operations. Therefore, Patent Publication No. 3503359 has a problem that even though the position of the distal end of the electrode tip in abutment has been measured beforehand, the position of the distal end of the electrode tip may deviate from the measured position.
Variations in welding pressure are also attributed to static friction in a mechanical unit of the spot welding gun. The influence of the static friction may not be known from a current flowing through the servomotor measured in Japanese Laid-open Publications No. 2008-296226 and No. 2010-000528. Therefore, there is a possibility that welding pressure corrected in Japanese Laid-open Patent Publication No. 2008-296226 and No. 2010-000528 do not reflect the influence of the static friction.
The present invention has been made under these circumstances and an object of the present invention is to provide a spot welding system that, when deformation or a crack has occurred in an arm of a spot welding gun or friction in a mechanical unit has changed due to wear in the mechanical unit, enables quick estimation of the occurrence of the abnormality in the spot welding gun.